The present invention relates to a method of manufacturing a thin-film transistor and a liquid crystal display.
A thin-film transistor (TFT) in a liquid crystal display is composed of a silicon film and other films that are formed on an insulating substrate such as a glass substrate, and used as a switching element provided for a pixel of the liquid crystal display or a driver element in a peripheral circuit. As a silicon film for a TFT, an amorphous silicon film is used in many cases. In recent years, however, a polycrystalline silicon (hereinafter also referred to as simply xe2x80x9cpolysiliconxe2x80x9d) film that has higher characteristics has been used, and now is being developed increasingly.
Hereinafter, as an example of a conventional method of manufacturing a thin-film transistor, a method of manufacturing a top gate polysilicon TFT will be explained with reference to FIG. 6. The below mentioned method includes steps up to a step of depositing a gate insulating film.
First, an amorphous silicon film is deposited on the entire surface of a substrate 31, followed by crystallization to make the amorphous silicon film into a polysilicon film 32 (FIG. 6(a)). Then, the polysilicon film 32 is subjected to patterning into a predetermined island pattern so as to be formed into a semiconductor layer of a transistor. This patterning generally is carried out as follows. First, a photoresist is coated on the polysilicon film 32 and dried, followed by exposure and development to form a predetermined resist pattern 34 (FIG. 6(b)). Next, the polysilicon film 32 is etched partially away by using the resist pattern 34 as a mask (FIG. 6(c)). Thereafter, the resist pattern 34 is removed by, for example, immersing it in a peeling solution (FIG. 6(d)). A SiO2 film 35 as a gate insulating film is deposited so that it covers the thus obtained polysilicon film 32 with a predetermined pattern (FIG. 6(e)).
Furthermore, in the method of manufacturing the polysilicon TFT, a process in which impurities directly are implanted into the polysilicon film may be used. A conventional example of this process will be explained with reference to FIG. 7.
As in the above-mentioned conventional example, a polysilicon film 32 is formed in an island form on a substrate 31 (FIG. 7(a)). Impurities such as phosphorus (P) are implanted selectively into predetermined regions on the polysilicon film. First, a photoresist is coated on the polysilicon film 32 and dried, followed by exposure and development to form a resist pattern 34 on a region into which impurities are not implanted (a region to be formed into a channel) (FIG. 7(b)). Next, impurity ions 38, for example, P ions etc. are implanted into the polysilicon film 32 by using the resist pattern 34 as a mask so as to form a region into which impurities are implanted (regions to be formed in source and drain regions) 37 (FIG. 7(c)). Thereafter, the resist pattern 34 is removed, by, for example, immersing it in a peeling solution (FIG. 7(d)). A SiO2 film 35 as a gate insulating film is deposited so that it covers the thus obtained polysilicon film 32 (FIG. 7(e)).
When a polysilicon TFT is formed in the method shown in FIGS. 6 and 7, polysilicon inevitably is exposed to the peeling solution in a step of peeling off the resist, and the surface of polysilicon may be chemically deteriorated or may be etched slightly due to alkaline components in the peeling solution. Furthermore, a subsequent step may proceed with the peeling solution or components of the resist remaining on the surface of the polysilicon. As a result, there arises a problem in that deterioration of the TFT properties, for example, a shortage of ON-state current, variation of threshold voltage, may be caused. In particular, it is desired that a thin-film transistor used for a liquid crystal display secures the reliability of products by suppressing the above-mentioned deterioration of a TFT over the entire region of a large-sized substrate.
It is therefore an object of the present invention to provide a method of manufacturing a thin-film transistor excellent in characteristics and reliability and also to provide a liquid crystal display manufactured by using this method.
The method of manufacturing a thin-film transistor of the present invention includes: bringing a surface of a semiconductor film formed on a substrate into contact with ozone-containing water to form a surface-oxidized layer on the surface, forming a predetermined pattern mask on the semiconductor film directly or via another film, carrying out either of processes selected from etching and impurity ion implantation by using the mask, and removing the mask with the surface-oxidized layer being formed at least on an exposed portion of the surface of the semiconductor film.
With the manufacturing method of the present invention, since the surface-oxidized layer is formed, the semiconductor film is not exposed directly to the peeling solution. Therefore, it is possible to suppress the deterioration or erosion of the semiconductor film. It also is possible to suppress the components of the mask or the peeling solution from remaining on the semiconductor film. Furthermore, since the manufacturing method of the present invention uses the ozone-containing water, it is possible to oxidize the surface of the semiconductor film rapidly and uniformly. Therefore, the manufacturing method using ozone-containing water of the present invention is particularly suitable for manufacturing a liquid crystal display including a large-sized substrate.
In the above-mentioned manufacturing method, the surface-oxidized layer may remain. However, it is preferable that the above-mentioned method further includes removing the surface-oxidized layer after the mask is removed. This is because even if contaminants such as the above-mentioned components remain on the surface-oxidized layer, the contaminants can be removed together with the surface-oxidized layer.
The above-mentioned manufacturing method further may include removing a surface layer of the semiconductor film before the surface-oxidized layer is formed. If contaminants are removed in advance, the surface of the semiconductor film further can be kept clean.
The thickness of the surface-oxidized layer is suitably in the range from 0.5 nm to 5 nm, and particularly suitably in the range from 1 nm to 5 nm. When this thickness is too thin, the effect of protecting the surface cannot be obtained sufficiently. On the other hand, when the film thickness of the surface-oxidized layer is too thick, although the effect of protecting the surface of the semiconductor film can still be obtained, there may arise a problem in that the other exposed surface such as a glass surface, may be etched in the step of removing the surface-oxidized layer.
As a pattern mask, a photoresist is used in many cases. In this case, for removing the photoresist, an alkaline peeling solution may be used.
As the semiconductor film, in general, at least one selected from a polycrystalline silicon film and an amorphous silicon film is used. However, when the present invention is used particularly for a polysilicon film, a greater effect can be obtained. This is because the polysilicon film is more excellent in the electric property than an amorphous silicon film, but it is susceptible to the surface contamination.
The typical step in which the present invention preferably is used is a process (etching or impurity ions implantation) of a semiconductor film with a mask. However, the present invention is not necessarily limited to these and similarly can be used for any other step when the step requires removing a mask with the surface of a semiconductor film exposed. For example, in the step in which a mask with a predetermined pattern is formed on a semiconductor film via at least one insulating film and the insulating film is etched by the use of this mask, the surface of the semiconductor film is exposed by the etching of the insulating film. Therefore, also in this step, in the step of removing the mask after etching, a surface of the semiconductor layer may be brought into contact with ozone-containing water to form a surface-oxidized layer at least on an exposed portion of the surface of the semiconductor layer.
Thus, in the manufacturing method of the present invention, the mask may be formed directly on the semiconductor film. However, the present invention is not necessarily limited to this, and one or two or more layer(s) may be interposed between the semiconductor film and the mask. Furthermore, the surface-oxidized layer may be formed at any time before the mask is removed. In some embodiments, the surface-oxidized layer is not necessarily formed before the mask is formed.
The liquid crystal display disclosed in the present invention includes a thin-film transistor manufactured by using the manufacturing method of the present invention. This liquid crystal display generally includes a thin-film transistor array disposed on a glass substrate, reflecting excellent characteristics and reliability of the thin-film transistor obtained by using the manufacturing method of the present invention.